Meaning
Fluid behavior and flow dynamics of liquid encapsulants during the dispensing and capillary flow processes determine the uniform, void-free filling of the narrow gap between a silicon die and its substrate. The underfill rheology is characterized by temperature-dependent viscosity, surface tension, and contact angle, which collectively govern how the material flows under the chip. This discipline applies to advanced flip-chip assemblies, ball grid array packages, and wafer-level packaging operations.
It does not apply to solid molding compounds that are transfer-molded or compression-molded under high external pressure.
Capillary Flow
The flow rate is driven by the surface tension of the underfill and the capillary force in the narrow gap. To achieve optimal underfill rheology, the material must exhibit low viscosity at the dispensing temperature, which allows it to flow quickly and evenly. This prevents the formation of voids that can lead to solder joint failures.
Temperature Dependence
Viscosity is highly sensitive to temperature and decreases initially before polymerization begins. If the dispensing substrate is too hot, the underfill rheology is affected, causing premature gelation and incomplete flow. This thermal sensitivity requires precise control of the dispensing temperature on the production line.
Filler Particle Effect
The addition of silica particles to reduce the coefficient of thermal expansion increases the complexity of the fluid behavior. In these filled materials, underfill rheology can exhibit non-Newtonian shear-thinning behavior, which must be carefully managed to avoid filler settling or separation during dispensing. This maintains a uniform material composition throughout the package.
